The ATM technique is of increasing importance for the transport and switching of digital informational flows at very high speed, such as those present in wide band integrated service digital networks. In this technique, the information associated with the various services is organized into contiguous packets of fixed length (about 400 bits), called "cells", formed by an information field and a header field (tag) that carries service information, including the information necessary for the routing through the network.
In an ATM network, the switching nodes must carry out both routing the cells (thus performing a spatial switching function), and avoidance of possible conflict situations, which occur when several cells entering the node simultaneously from different inputs have to be routed toward the same output (thus, the mode performs a memorization function). In order to satisfy the requirements for bit rate and bandwidth of modem integrated services networks, ATM switching nodes based on optical techniques have been proposed. There, both routing and memorization are performed by means of optical devices. In this way, it has been possible to operate at very high bit rates, of the order of several Gbit/s.
An example of a node of this type is described, for instance, in commonly owned European patent application EP-A-0 411 562.
In a communication system based on the ATM technique, the cells containing information may be emitted by the various transmitting stations at random times. In the optical systems proposed so far, there is the need for a synchronization of the cells arriving at the various inputs of a node, so as to facilitate its operation. In a large optical switching node, comprising a multi-stage network, there may be a further synchronization need, due to the uncertainty about the length of the optical paths that the cells travel through in the node. In the first case, the cell synchronization system must be able to recover time shifts of .+-. half a cell, whereas in the second case the time shift to be recovered may be of much smaller proportions, in the order of a few nanoseconds.
The synchronization of the ATM cells arriving at the different inputs of a single-stage optical switching node is described in European patent application EP-A 0 411 562 mentioned above. The time realignment is performed by acting on signals converted into electrical form, within the devices performing the tag processing necessary for routing. The possibility to operate directly on optical signals is also suggested.